Tile Tub/Shower Leak

A few days back, I posted here because I was convinced I had a leak with my tub door. After a shower, water would leak into the subfloor/basement in two areas (at each outside corner of the tub). Well, I installed a new door and I still have the same problem. I eliminated the door from the equation entirely by putting in a shower curtain/rod/liner and still have the leak. I am hoping someone here might be able to help me solve this mystery leak.

I feel pretty confident that I ruled out the supply pipes and drains. This is because I tried redirecting water from the shower head directly to the tub drain (by wrapping a liner around the head) and that worked OK with no leaks. I can take a bath with no problem and the tub looks in good shape, so I don't think there are any leaks in the tub. I have tile walls and have cleaned and re-caulked the wall/tub joints three times over and have the same problem. I've been super-thorough about following good practices with caulking so I'm skeptical it has anything to do with the caulk and the tub/wall joints. I can't get it to leak pouring water over the joints.

Interestingly, I start to get the leak if I wrap the liner around the walls to redirect water into the tub only without hitting anything else directly. Since the tub doesn't appear to have a leak, the only difference I can note is that this test allows the steam to collect on the tile walls. In both this test and in practice, I don't see the leak until after about 20 minutes of usage (my wife and I tend to take showers after each other).

My best theory at the moment is that this has something to do with the steam collecting on the tiles and possibly leaking through hairline cracks in the grout, etc. Could this be happening? This problem seems to be eliminated/minimized by allowing the fan to run for a long time between showers. The grout looks to be in pretty good shape, but there appears to be some hairline cracks. If this could be the issue, would replacing the grout fix this problem, or is this representative of a bigger issue if this is allowed to happen? Any other thoughts?

In a properly installed shower, it should not leak even without any tile on the wall...the tile are considered a decorative and wear surface, not the waterproofing. First thing I'd check is to see if the tub is level end to end and side to side. Any idea if this was a drop-in tub or one with a tile flange?

You could have a leak between the overflow outlet and the gasket, or at the bottom of the shoe that holds the drain.

Regarding the overflow outlet and drain, I luckily do have full view of those components from an access panel / the basement and there's no leak there.

I think you're on to something regarding the flange/level!

I cut out the caulk at one of the failing corners, looked into the joint with a flashlight and saw that the tub has a flange past the tile. I have noticed that the tub is just slightly uneven. If I use a level, the bubble still appears in the center window but is just slightly offset. I am not sure if this is within an acceptable range or not. The slope is probably slightly lower near the side of the tub that the door would be on (also where my two leaking corners are).

Does it sound likely at this point that the water is getting behind the tile, collecting at the base between the flange and caulk, and the eventually flowing down that channel to the corners, and leaking where the flange stops at the corners? This would potentially explain so much.

If I understand this to be the problem, what part of this system/construction is failing? How is water that gets passed the tile supposed to escape if there's no outlet to a drain?

If the tub is level, then anything that gets behind the tile (and not much should) tends to flow out into the tub. If there's a slope, it will pool, and once saturated, it will flow. If it is totally saturated, then the caulk will have trouble sealing anything and if there's any food there (soap scum, body oils, dead skin, etc.), it'll grow mold.

If the base is caulked all around, and water gets trapped in the channel between the flange and caulk, how would that water escape to the tub? I'm not sure how it would find its way back to the tub if it's blocked in by caulk. I am assuming that it's pooling and eventually building up to the point where it finds an escape out of the corners where the flange ends. Is the problem that I don't have weep holes in the caulk?

The only water that should get back there is if you have a crack in the grout. The grout will get damp from the front, and will wick a little in, but unless the shower is used lots, it will dry out to the front. So, there should not be any liquid back there. Weep holes in the caulked tub/tile junction is an often debated topic. The industry standards call for it to be caulked (FWIW). If the caulk has failed, then it can get back there and pool, saturate the cbu (and maybe the tile - porcelain is pretty impervious, but common ceramic can absorb a fair amount), and eventually flow if it builds up rather than dries out between showers. If the tub isn't level, it can just end up flowing, and not necessarily from behind the tile (unless the caulk has failed).